C Macquarie Catchment

C MACQUARIE CATCHMENT 1.0 Hydrology The upper Macquarie River catchment is one of the driest areas in the state as it lies in the rain shadow of both westerly and easterly weather systems. Townships such as Campbell Town and Ross have historically experienced hardships in terms of the available supply of water and it is for this reason that man-made storages have been constructed. Lake Leake and Tooms Lake were created largely to meet township requirements for water as well as stock and domestic requirements downstream, but they also provide irrigation relief from the ephemeral (i.e. intermittent) nature of the streams in these areas. It is not unusual for there to be effectively zero flow in this region during summer months. The main river system is also ephemeral. Long "broadwaters" or ponds form in the river reaches and these have been assessed to be vital for the maintenance of the local aquatic ecosystems. 1.1 Historical Background Records on flows in the Macquarie River are surprisingly sparse given the attempts to develop the area. The longest periods of record appear to be related to Lake Leake and Tooms Lake. These records do provide some overall picture of monthly catchment yields, however, they also include the effect of evaporation on the storages and hence tend to under- estimate true runoff. Tributary Station Name Station Period of Record Catchment Number Area (Km2) Macquarie River Macquarie downstream Longmarsh Dam 18210 1975 - 1990 89.7 Tooms Lake outflow 18206 1973 - 1992 60.2 Macquarie at Trefusis 18217 1979 - 1992 365 Elizabeth at Lake Leake 9 1922 - 1988 69.7 Macquarie downstream Elizabeth River 18312 1989 - 1995 2041 Lake R. downstream Woods Lake 165 1955 - 1991 347 Poatina Discharge 502 1964 - 1991 - + Sites with altered catchments due to water diversions for irrigation schemes or HEC operations. 1.2 Catchment Yields and Distribution of Flows Excluding diversions from Great Lake, the Macquarie River catchment provides about 28% of the natural annual flow at Launceston. Annual Yields Figures 1.1a to 1.1d show the average annual, average winter, and average summer flows at key monitoring sites throughout the catchment. All sites have high variability in average flows from year to year. Flows in the upper catchment above Ross have been measured at the site of the proposed Long Marsh Dam, at Trefusis and on the Tooms River downstream Tooms Lake. The difference in flows at Trefusis and in the Tooms River is effectively the flow coming from the upper catchment. 160 161 162 163 164 Inspection of Figures 1.1a to 1.1c shows that there is a high correlation between flows at the three sites especially on an annual average basis and during high flow months. However the effect of Tooms Lake regulation on flows at Trefusis is clearly evident in summer. Flows in this area are highly variable in both winter and summer. Further information on the regulation of Tooms Lake is provided in Section 1.6. Figure 1.1d provides a graphical summary of releases (and spill) from Lake Leake. Again high variability in both winter and summer flows is seen with a relatively consistent base level of flows from the lake during summer. Further information on the regulation of Lake Leake is provided in Section 1.6. Flows down the Lake River are now regulated by releases from Woods Lake. The Hydro- Electric Commission is currently reviewing these measurements and hence the data was unavailable for analysis. Monthly Yields The variability of monthly flows in the Macquarie River catchment is shown in Figures 1.2a to 1.2e which provide box and whisker plots for each of the study sites. Box and whisker plots for Longmarsh and Trefusis show high variability in the occurrence large flows throughout the year, longer records may realise more information on the occurrence of these events. These plots are highly seasonal with peak median flow occurring in July. The ephemeral nature of the river is seen during the December through May period. Outflows from Tooms Lake are clearly noticeable in the November through April period when the median flows in the Tooms River “jump” above the normal trend. Once again the occurrence of high flow outliers is wide spread throughout the year. Interestingly the peak median flow occurs in July and/or August suggesting some lag in releasing water from the lake through replenishment of the storage capacity. Regulation of Lake Leake is also evident in the box and whisker plot of monthly flows. In this case the period December through March seems to be the dominant period for releases. In general releases and spills from Lake Leake seem to be more variable than those from Tooms Lake. Peak median flows from the lake occur in August and September which is surprising since Tooms Lake has a larger storage capacity than Lake Leake. 1.3 Floods A substantial length of record is required to perform flood frequency analysis. In the Macquarie River catchment the only site at which flood frequency analysis was possible was Macquarie at Trefusis; and only after substantial theoretical corrections were made to account for years with no flooding (ie peak flows <= 100 cumecs). For this reason the calculation of flood probabilities outside the general cluster of observations should be treated with care (Figure 1.3). 1.4 Droughts and Low Flows Low flow frequency curves have been derived for a range of durations from 1 day through 90 days (Figures 1.4a to 1.4f). The curves give the probability that any given minimum flow will occur over various time periods. For example, over five days the probability that a minimum average daily flow of about 0.25 cumecs will occur is approximately 90%, while over a longer period such as ninety days this probability decreases down to around 25%. 165 166 167 168 169 170 171 1.5 The Impacts of HEC Regulation Over the last 30 to 40 years the HEC has had a substantial effect on flows in the middle to lower reaches of the Macquarie River catchment: 1962 Arthur Lakes dammed to form Arthurs Lake 1964 First release from Poatina P.S. 1965 Lake River dammed to form Woods Lake 1966 Arthurs Lake water diverted into Great Lake. Currently Arthurs Lake does not release water down the Lake River unless absolutely necessary. The majority of the water is pumped over the hill into the Great Lake catchment where it passes through Tods Corner power station and eventually back into the Basin via the Poatina power station. Woods Lake provides a regulated flow in the Lake and lower Macquarie Rivers to provide riparian, stock and domestic requirements to prescriptive right holders. Substantial volumes of water are diverted from the Lake River into Great Lake, but generally much more water is released into the South Esk Basin via Poatina than is diverted through Tods Corner Canal. This is due to the diversion of water from the top of the Ouse River catchment into Great Lake via Lake Augusta and the Liawenee canal. As a broad guide to the amount of water diverted into the Basin the two power station discharges have been differenced and are presented in Figure 1.5, negative figures indicate a net loss to the Basin, positive figures a net gain. Poatina usually provides around 60 to 70% of the flows experienced at Launceston during the summer months. This percentage decreases markedly during winter and starts to increase once more circa September each year. These releases from Poatina cause increased flows in the lower Maquarie River during summer. 1.6 Irrigation Storage Lake Leake Lake Leake was constructed after much debate and enterprise during the later half of the 19th century finally being closed in 1883/1884. The 5 metre dam has a storage volume of some 18.90 Mm3 and a surface area of 6 Km2. Although it is believed that storage levels have been collected since the closure of the dam the only records available at this point in time are for a short period from 1922 - 1928 and from 1946 to date. The behaviour of the lake for the latter period is displayed in Figure 1.6, the solid horizontal lines represent the onset of spill at the lake and this level was altered circa 1971/1972 when the spillway crest was raised by seven (7) inches. The figure shows that Lake Leake has not failed in this 50 year period, but is maintained at or above about half full volume. This is surprising given the relatively small size of the storage and the severity of the drought periods experienced in that time. However, the operating criteria for the lake are not based solely on the allocation of water downstream, water is effectively allocated within storage to maintain the lake as a popular fishing and recreational attraction. 172 173 174 175 Tooms Lake Tooms Lake is a larger storage than Lake Leake having a capacity at Full Supply Level of some 24800 ML. Data on the behaviour of the lake is relatively sparse. Collection and processing of some intermittent records held by the Ross Council does however, indicate that, in broad terms, the behaviour of Tooms Lake is not unlike that at Lake Leake (Figure 1.7). 2.0 Water Quality 2.1 Historical Background The Macquarie River drains approximately 3860 km2 and incorporates the Lake River, the Elizabeth River, Tooms River and the Blackman River. Flows in the Upper Macquarie, the Elizabeth and the Lake River are regulated through releases of water from impoundments in the headwaters. Flows in the lower reaches of the Macquarie River are further greatly increased during the summer through discharge from the Poatina Power Station which generates hydro-electricity from water from Great Lake on the Central Plateau.

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